Difference between Much and More?

The word “more” is used when you want to tell the reader that there’s a greater degree of something in comparison. For example, in your sentence “People who have completed engineering have more chances of getting selected,” you are saying that people who have completed engineering have more chances of getting selected than “people who have not” completed engineering.

Your use of “much” in both sentences is grammatically incorrect. It’s not correct to say “I think compared to java, PHP has much demand.” When you’re comparing the two, you should use the word “more” and say “I think compared to java, there is more demand for PHP.” Use “more” when you’re comparing two things. If you use the word “much”, write two separate sentences, like this—- “There is much demand for PHP. I think compared to java, there is more demand for PHP.” Or, you can write “I think compared to java, there is much demand for PHP.” Instead of writing “PHP has much demand,” you need to re-arrange the sentence and add “there is” and “for”.
Grammatically, that’s okay, but you are not saying that there is a “greater” demand for PHP. You are only telling the reader that there is a lot of demand for PHP.
In the first sentence, you are telling the reader that there is a lot of demand for PHP, you are not “comparing” it to something else. You are quantifying it. In the second sentence, you are comparing two things, so the use of “more” is appropriate.

In your sentence about engineering, it is not grammatically correct to say “People who have completed engineering have much chances of getting selected.”

Instead, you should say “People who have completed engineering have MANY chances of getting selected,” or “People who have completed engineering have a lot of chances of getting selected.”

Your other sentence, “People who have completed engineering have more chances of getting selected” is grammatically correct.

You can write it like that and leave it, or if you want to compare it to something else, write “People who have completed engineering have more chances of getting selected than people who have not.”

Again, always use “more” when comparing, and “much” when telling the reader the amount of something.

Yeah, what ↑he↑ said….

Much is used to describe a greatness of an amount. More is used to describe an additional amount to that which was previously existent.

More is the comparative, used when simply adding er doesn’t sound right.

Big, bigger biggest. Handsome, more handsome, most handsome.

Much means a lot.

Much ado about nothing.

And there’s always “I need much more” ^^

Usually the confusion is between “much” and “many”. They are both words of “indeterminate quantity signifying abundance”, but beginning users often confuse when to use them.

We use “many” when we have a number of quantifiable objects: many people; many houses; many ships; many leaves on the tree.

We use “much” when we have an abundant quantity, but it’s not discretely countable: much soup; much water (we could say “many gallons of water”, if they were counted or separated, for example); much dirt.

But in the question you asked, “more” just refers to a relative abundance over some other amount. Mary has much soup; I have more. Canada has a large population; the US has more. The US has a lot of dirt; Canada has more.

“much” with noncountable nouns and “many” with countable nouns

@MRSHINYSHOES : Really a good answer. :)